This invention relates to opto-electronic reference generators and, more particularly, to an arrangement for providing a constant level output signal therefrom.
As described in U.S. Pat. No. 4,621,529, guidance systems for modern aircraft require rate and acceleration sensing. This may be achieved by utilizing a gyroscopic multi-sensor assembly which operates by sensing an angular velocity about some axis perpendicular to the spin bearing axis of the device with the use of a gyroscopic element. The gyroscopic element is mechanically restrained by a piezoelectric crystal beam so that gyroscopic reaction to angular velocity mechanically strains the piezoelectric crystal beam which in turn produces an electrical output signal proportional to angular velocity input. Similarly, a restrained piezoelectric crystal beam can be employed to sense linear acceleration along any axis perpendicular to the spin bearing axis. Thus, through the use of one or more mechanically restrained piezoelectric crystal beams, both the angular velocity and acceleration amplitude may be converted into electrical signals.
The spin bearing axis discussed above is defined by the rotational axis of the output shaft of a motor, upon which output shaft the gyroscopic element is mounted. In order to provide a reference signal which is required for timing purposes, a reference signal generator is also coupled to the motor output shaft. Such a reference signal generator may illustratively include a disk mounted for rotation on the output shaft, which disk is diametrically divided into a first region which is light transmissive and a second region which is opaque. The disk is then straddled by an optical sensing unit which includes a light emitting diode on one side of the disk and a phototransistor on the other side of the disk. Ideally, this sensor provides a square wave output.
While the aforedescribed arrangement operates generally satisfactorily, it will be appreciated that when mounted in an aircraft, the multi-sensor assembly is subjected to varying temperatures. Since the outputs of the light emitting diode and the phototransistor vary with temperature, such temperature variation results in a variation in the amplitude of the square wave output. In the particular system with which the present invention is concerned, the output of the phototransistor is provided as the input to a comparator circuit. There is a significant rise time at the output of the photo-transistor so that if the amplitude of the output varies, the relative triggering time of the comparator circuit will likewise vary. This is an undesirable condition.
It is therefore a primary object of the present invention to provide an arrangement wherein the amplitude of the phototransistor output signal is maintained at a substantially constant level independent of temperature.